Many biological functions are carried out by regulating the expression levels of various genes, either through changes in levels of transcription (e.g. through control of initiation, provision of RNA precursors, RNA processing, etc.) of particular genes, through changes in the copy number of the genetic DNA, through changes in RNA processing such as polyadenylation and splicing or RNA stability or through changes in protein synthesis. Changes in the expression levels of particular genes (e.g. oncogenes or tumor suppressors), serve as signposts for the presence and progression of various diseases.
Gene expression analysis using microarrays allows study of entire expressed genomes and has led to insights into the involvement of diverse molecular interactions as well as the pathologies that result from their disruption. Microarrays have been used to distinguish between pathologically similar diseases such as Acute Myeloblastic Leukemia and Acute Lymphoblastic Leukemia, Golub et al. Science, 286 (5439), 531-7 (1999), to provide insight into inflammatory diseases such as psoriasis and systemic lupus erythematosus (SLE), see for example, Zhou et al. Physiological Genomics, 13(1), 69-78, (2003) and to study diseases such as diabetes, Mootha et al. Nature Genetics, 34, 267-273 (2003).